The present invention relates generally to modem technology, more particularly to PCM modem technology and, more particularly, to a method of compensating for systemic impairments in a telecommunications network.
PCM modem technology as recently proposed requires that no changes need be made in the existing telecommunications system. Much attention has been paid to impairments affecting the local analog loop. However, there are several known digital impairments which can equally affect the performance of the PCM modems. The known impairments are the following:
a) Digital attenuation pads (3 dB and 6 dB)
b) Robbed bit signaling
c) PCM sample slips
d) PCM code conversions
It would be impractical to require the telephone carriers to change their transmission facilities. These digital impairments can and should be accommodated by PCM modems. Each of the above-identified impairments are further discussed herebelow.
Digital Attenuation Pads
The network model diagrams for evaluation of PCM modems described in Sierra Semiconductor, xe2x80x9cLiaison to TR30.1 concerning PCM modemsxe2x80x9d, Ft. Lauderdale, Jan. 23-24, 1997, Doc. FL-08 and Intel: xe2x80x9cProposed Provisional PCM Modern Network Modelxe2x80x9d, Ft. Lauderdale, Jan. 23-24, 1997, Doc. FL-02 show configurations with 3 and 6 dB pads within the digital transport/switching network.
FIG. 2 presents the network model of the Intel paper, which illustrates the two-wire analog to four-wire digital PCM interface. Included in the interface are the A/D and D/A converters that process input and output of the PCM modem, analog attenuation elements including IMD and RTD/AD/EDD elements, noise generators summed with transmitted signals and a balance network interface.
G. 121 further identifies additional attenuation pads used by various countries outside of North America. These pads are implemented via digital circuits or ROMs which provide a systematic remapping of PCM codes to other PCM codes. This mapping to a new space in the digital network is invariant for a telephone connection.
Robbed Bit Signaling
Robbed bit signaling is used on North American telephone trunks for interoffice signaling. The least significant bit of every sixth PCM sample is used for signaling. Within the telephone network from end-to-end, typically two PCM samples within every six may be used for signaling, although in the worst case it may be all six. The effect of this impairment is the inability to convey information in this bit position. It further may affect PCM code transformations due to attenuation pads or law conversions.
PCM Sample Slips
PCM sample slips occur for purposes of synchronization within the telephone network or between telephone networks. Either a PCM sample is completely dropped or repeated. Detection of such impairments relies on redundancy of trellis coding or forward error correction of some form.
PCM Code Conversions
PCM code conversions between xcexc-law and A-law occur over international calls. This is a systematic mapping of codes from one law to the other. This mapping is not exactly one-to-one according to the recommendations in G.711.
Regulatory Analog Transmission Power Limits
In addition to the digital impairments, telecommunications regulatory agencies have established power limits for circuits which terminate in an analog local loop. Communications Certification Laboratory: xe2x80x9cLiaison to T1A1.7, and TR30.1 on FCC Part 68 compliance of PCM Modemsxe2x80x9d, Ft. Lauderdale, Jan. 23-24, 1997, Doc. FL-10. This limitation is a result of real implementation restrictions already within the deployed telephone equipment.
The highest data rates in the downstream direction (from the digital to the analog modem) can only be achieved if all of the xcexc-law encoded levels can be used. However, the average energy of a sequence that uses all 255 xcexc-law levels with equal probability is well above the maximum permitted transmit power level of xe2x88x9212 dBmO specified in FCC part 68.
Thus it is apparent that the need to comply with this limitation requires use of a subset of the available PCM codes in the set of PCM symbols to be used for the high speed data transmission. Non-equally probably mappings (i.e. shell mapping) could be used to alter and control the probability distribution of each of the PCM codes in the set of symbols. Even with non-equally probably mapping techniques, the analysis in RSA Communications: xe2x80x9cxcexc-Law Signal Powerxe2x80x9d, Ft. Lauderdale, Jan. 23-24, 1997, Doc. FL-02 suggests that both the highest and lowest segments of the xcexc-Law codes would not be used. The highest segments are eliminated because of signal power limits. The lowest are eliminated to increase the minimum distance between signal values.
What is needed, then, is a method of compensating for the above-described impairments in a telecommunications network.
Embodiments of the present invention provide a method of compensating for systemic impairments in a telecommunications network, comprising transmitting a candidate set of digital symbols from a digital modem to an analog modem, processing an analog representation of the candidate set of digital symbols by the analog modem, deriving a set of data information symbols from the analog representation of the candidate set of digital symbols, transmitting a representation of the data information symbols by the digital modem for subsequent transmission of data from the digital modem.
Further embodiments of the invention provide a method of compensating for systemic impairments in a telecommunications network, comprising sending digital symbols from a digital modem to an analog modem, converting the digital symbols into analog signals, processing the analog signals by the analog modem, training a receiver of the analog modem to recognize changes in the analog signals that correspond to known changes in digital symbols from the digital modem to the analog modem, processing an analog representation of the candidate set of digital symbols by the analog modem, deriving a set of data information symbols from the analog representation of the candidate set of digital symbols, transmitting a representation of the data information symbols from the analog modem to the digital modem, and, using the representation of the data information symbols by the digital modem for subsequent transmission of data from the digital modem.